1. Field of the Invention
The present invention relates to an optical information processing apparatus capable of writing or reading various information into or from a recording medium by irradiating a light beam to said recording medium.
2. Description of the Prior Art
In the field of optical information processing apparatus there are already known optical disk apparatus, magnetooptical disk apparatus etc. As an example, in case of a magnetooptical disk apparatus, the signal regeneration from the record is conducted in the following manner.
The recording medium is composed of so-called magnetooptical disk consisting of a disk-shaped substrate of glass or plastic and a vertically magnetizable layer of a thickness of several microns provided on said substrate. Said vertically magnetizable layer is composed for example of an amorphous alloy and can be magnetized in a direction perpendicular to the layer surface.
At the information recording on such magnetooptical disk memory, the vertically magnetizable layer thereof is uniformly magnetized in advance in a direction, and is irradiated by a laser beam spot digitally modulated by the information signal to bring said layer to a temperature above Curie point, whereby the magnetization in the irradiated area is inverted under the influence of an external magnetic field to form a record bit representing a logic signal "1" or "0".
At the reading of the information thus recorded on the magnetooptical disk memory, a light beam spot for signal reading irradiates the vertically magnetizable layer, and the signal is read by means of Kerr effect by which the direction of polarization of the reflected beam is affected by the direction of magnetization in the vertically magnetizable layer.
In information recording and regeneration in the above-mentioned optical information processing apparatus, a tracking control is indispensable for constantly following the information track on the recording medium precisely with the light beam. FIG. 1 shows the function of conventional tracking control.
In FIG. 1, a disk-shaped recording medium, hereinafter simply called disk, rotates about a spindle 2. A light beam L from a laser beam source 3 is split, in a diffraction grating 4, into a main beam L2 and subsidiary beams L1, L3. Said beams L1-L3 are guided through a lens 5 and a half mirror 6, then reflected by a tracking mirror 7 and are projected through an objective lens 8 onto the disk 1. FIG. 2 shows the beams L1-L3 on the disk. The information track 15 on the disk 1 is composed of an array of record bits 16 which are the smallest details in the optical structure. The main beam L2 forms a spot S2 on the information track, while the subsidiary beams L1, L3 form S1, S3 on different positions in the transversal direction of the information track.
The beams L1-L3 projected onto and reflected by the disk 1 are again guided through the objective lens 8, then reflected by the mirrors 7, 6 and respectively supplied to photosensors 9, 10, 11 for conversion into electrical signals. The signal from the photosensor 10 is supplied to a reproducing circuit 12 to provide a demodulated signal at a terminal 13. Also the output signals of the photosensors 9, 11 are processed in a servo circuit 14 to provide a tracking signal, according to which the tracking mirror 7 is rotated to deflect the beams L1-L3 in the transversal direction of the information track 15, thus constantly guiding the main beam L2 exactly onto the information track.
However the conventional optical information processing apparatus is associated with a drawback of unstable tracking because of erroneous operation frequently caused by small scars or dust on the disk, as the spot formed by the subsidiary beam is substantially circular and is of a size corresponding to the record bit.